How can you get to Blaine in Pokemon HeartGold?

he is at the seafome islands because his gym got destroyed on cinnabar island . when u enter the seafom island he will be up the ladder inside
If you've been to Cinnabar Island yet, you will see that there is a notice there. It says that Blaine relocated his gym to the Seafoam Islands. All you have to do is go into the first island that you get to.
Blaine is on Seafoam Island, which is east of Cinnabar.
Seafoam island (where Articuno is)
On Cinnabar Island, head East to Route 20. Keep going until you come to seafoam islands. Go in and you will see a ladder going up in the middle of the room. Go up the ladder and it will lead you to the new Cinnabar Gym. Blaine is inside.

Inside the cave there are two ways, to Articuno and to Blaine's Gym

You meet him on cinnibar island and then go to his gym.

He's in the Seaform islands because cinnabar was destroyed by a volcano.